1. Field of the Invention
This invention relates generally to fluid dynamic bearings and more specifically to a fluid dynamic bearing assembly configured with an orbital ring that rotates at a fractional speed, thereby increasing the overall stiffness-to-power ratio of the assembly.
2. Description of the Background Art
FIG. 1 is an exploded perspective view illustrating a prior art magnetic disc drive 100. As shown, magnetic disc drive 100 may include, without limitation, a housing 105, a shaft 130, discs 135 and a suspension arm assembly 150. Housing 105 includes a base 110 that is attached to a cover 115. In addition, a seal 120 may be disposed in between base 110 and cover 115. Discs 135, which have surfaces 140 covered with a magnetic media configured to store information magnetically, are coupled to shaft 130. During operation, suspension arm assembly 150 is configured to suspend read/write heads 145 above surfaces 140 as a fluid dynamic bearing motor (not shown) rotates discs 135 about shaft 130 at high speed. Suspension arm assembly 150 is further configured to move read/write heads 145 radially across surfaces 140 to position read/write heads 145 above different radially spaced tracks (not shown) disposed on surfaces 140 where magnetically encoded information may be stored within the magnetic media. Once positioned, read/write heads 145 may either read magnetically encoded information from or write magnetically encoded information to the magnetic media at selected locations.
FIG. 2 is a cross-sectional view illustrating a prior art fluid dynamic bearing motor assembly 200. As shown, a central fixed shaft 250 has a thrust plate 252 disposed on one end. Shaft 250 is fixed relative to a sleeve 254. Thrust plate 252 sits within a recess 256 defined by sleeve 254. A counterplate 258 is attached to sleeve 254. At its other end, shaft 250 includes a shoulder 262, which rests on an upraised portion 264 of a base 266. A shaft extension 268 is attached to base 266. A hub 274 is supported on the outside surface of sleeve 254, and a magnet 276, also disposed on the outside surface of sleeve 254, is aligned with a stator 278, which is supported from base 266. Electromagnetic interaction between magnet 276 and stator 278 causes hub 274 to rotate. Hub 274 is configured to support one or more discs (not shown) as it rotates. Bearing fluid fills gap 277 between the surfaces of shaft 250 and the surrounding sleeve 254. Bearing fluid also fills gaps 279 and 281 between surfaces of thrust plate 252 and facing surfaces of sleeve 254 and counter plate 258. As is well known to persons skilled in the art, appropriate pumping grooves (not shown) are provided along one or more surfaces of gaps 277, 279 and 281 to maintain the fluid dynamic bearings that support hub 274 and sleeve 254 as they rotate.
Fluid dynamic bearings tend to generate less vibration and non-repetitive run-out in the rotating parts of motors than ball bearings and other types of bearings. For this reason, fluid dynamic bearing motors, such as fluid dynamic bearing motor assembly 200 described above in conjunction with FIG. 2, are oftentimes used in precision-oriented electronic devices to achieve better performance. For example, using a fluid dynamic bearing motor in a magnetic disc drive, such as magnetic disc drive 100 described above in conjunction with FIG. 1, results in more precise alignment between the tracks of the discs and the read/write heads. More precise alignment, in turn, allows discs to be designed with greater track densities, thereby allowing smaller discs and/or increasing the storage capacity of the discs.
As persons skilled in the art are aware, an ongoing challenge in fluid dynamic bearing motor design is balancing the tradeoff between motor performance and power consumption. On the one hand, increasing the stiffness of the fluid dynamic bearings results in less vibration in the motor's rotating parts and, therefore, increased motor precision and performance. On the other hand, however, increasing bearing stiffness results in greater power consumption because of increased viscous losses in the bearings. Conversely, decreasing the power consumption of the fluid dynamic bearings typically requires a substantial decrease in bearing stiffness and, hence, decreased motor performance.